John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are.

Sure. But the same Q would give wider BW at 2 MHz.
I have not ever done this, so I guess at what the final response could be.

But so what, I thought we were talking about IFs for audio here, not video
IFs? For an audio receiver I would think at the most we would want a 40
kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC
effectively limits the audio bandwidth to 10 kHz? What exactly do you see
as the advantage of a 2.0 MHz IF in an AM broadcast receiver?

Its a lot easier to get a wider pass band of 30 kHz with 2MHz IFTs than with
455 kHz IFTs.
Try it some time, and then you'll know.



The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the
bandwidth necessary for High Fidelity AM audio reception. The stage gains
will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of
similar bandwidth, with the exact stage gain depending somewhat on design
choices and practicalities. The wideband 455 kHz IF will have lower stage
gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from
lower stage gain. The wideband 455 kHz IF has the advantage that standard
RF front-end components like tuning capacitors and oscillator coils can be
used, while the 2.0 MHz IF will require special RF components.

What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

I suspect 3 x 2MHz IFTs would be easier to get a flat topped
pass band and sufficient steep roll off just outside the band.

I also suspect any old 455 kHz IFTs could easily
have about 3/4 of their turns removed, and retain the same
caps of 250pF.

For 250pF, to get 455 kHz, one needs 0.48 mH

For 250pF, and 2 MHz, one needs 0.025 mH.

To reduce L by 20 times, the turns would need reducing
by a factor of 1/4.47.

Thus the DCR would fall, and Q could rise.

I have used ex IFT windings with turns removed for
high Q RF input coils on my reciever, to get the range of
tuning required between 500 and 1750 kHz with a
20 pF to 360 pF tuning gang.
The ferrite slug is retained.

The wire is litz wire, with low RF resistance, hence it gives a high Q,
but for 2 mHz, solid round wire would probably be OK, like in 4.5 MHz
TV IFTs and 10.7 MHz FM IFTs.

HF IFTs are easier to wind than 455 kHz.



There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver?

Wide AF response was easily achieved.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios.

I differ. WW and what it became, Electronics World wasn't just an
amateur's magazine. It had cutting edge articles about electronics
from 1917 onwards, and I suggest you park yourself beside a
pile of all the old copies and have a good read.
Most of the info was only comprehensible by very well university educated
professionals, or intellectuals, and most ideas were backed up with mathematical
proofs which nearly all the general public couldn't understand.



I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

I leave you to your suppositions.



To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

The equality in performance depends on a large Q difference, with
544 kHz Q much lower than 2MHz Q to get the same BW.

Yes, although I have some reservations about the use of the term "Q", that
is obvious, but so what, what difference does it make?

Build a receiver, and find out.



The Q of a typical 455 kHz IFT is higher than you have indicated, because
the impedance of the LC circuit at Fo is required to be high to suit
pentode loading, and to get high gain.

You also are going to sacrifice stage gain in the same way with a 2.0 MHz
IF, so this is no more of a problem for the wideband 455 kHz IF than for
the 2.0 MHz IF.

Use more stages if stage gain is low.
The EA design used 3 IFTs, with two j-fet IF amps, with quite
heavily damped 455 kHz IF coils.




If the Q was real low, and hence the Fo impedance, you
would probably need 3 IFTs.

This is a consequence of the wide bandwidth, not the IF frequency, the
problem is identical at 2.0 MHz.

I have never tried 3 very damped IFTs.

The fact that you haven't tried something doesn't prove anything one way
or the other.

It means that what you or I am saying may not include all the facts about
the subject. Build and measure will give the facts.

Also, what does "damped" mean in this context?

Strapping resistance across the LC tuned circuit to reduce the Q.
The rate of attenuation just either side of the pass band becomes
much less, so more IF stages must be used.

I would

have to do some research, but I suspect that "damping" is more related to
filter bandwidth than to the center frequency, and both filters are aiming
for the same bandwidth.

Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.



What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.


Build a radio with 2MHz and measure it, maybe it works better.

You are the 2.0 MHz IF advocate not me, you still haven't suggested any
reason why it might work better from a bandwidth/selectivity standpoint?

I refuse to repeat myself any further.



Just don't knock the idea before trying it, or condemn the idea
with postulations about what might be.

I'm not, I know it would work, what I don't understand is what the
advantages are over a 455 kHz IF of the same bandwidth? You are not
explaining yourself, cite some concrete facts.

I have already stated that for a given Q, the pass band for a 2MHz IFT
is naturally a lot wider than for a 455 kHz IFT.

Put it this way, if you make IFTs of 100 kHz, then its all the harder to get
a flat topped bandpass response which is 20 kHz wide, with
high sloped skirt response each side.



These things must be tried and measured, to really know.

While I can't claim to have designed the filter I used, I have actually
built a transistor superhetrodyne AM tuner using a 455 kHz block filter
with a 30 kHz IF bandwidth.

Ceramic filters are another way to achieve the same bandpass filter
that the IFT could do.
But they were never used in tube sets for the BCB.


Will the 2.0 MHz IF work better than this?

I suspect yes, but getting a 2 MHz cermic filter with 30 kHz of BW might be
unobtainium.


Have you tried a properly designed wideband 455 kHz IF filter to see how
it worked?

Yes, and trying to squeeze 20 kHz of flat topped BW was difficult with stock IFTs.

I have already said what my solution was, to use a variable distance coils and
some damping
on IFT no1, which allowed me to have only 2 IFTs, and 1 IF amp, a 6BX6, fixed
bias,
for low thd IF amplification.

The filter I used came out of a 2-way land mobile radio and I
think it was about an 8 pole filter. Back in the old days of land mobile
here in the US, wider channels with greater bandwidth were used than are
used today. Over time the channels were squeezed down to accommodate
additional channels in the same space, and block filters of several
different bandwidths were available to suit the changing allocations and
operating frequencies.

I have also built wideband single frequency TRF receivers using modified
double tuned IF transformers.

One of the Electronics Australia kit designs I have used
a two stage TRF design with highish Q LC, with stagger tuning
at the low F part of the band. This utilised having mutual capacitive
coupling of the Ls in their earthy ends to ground via one common 0.1 uF.
I couldn't easily reproduce the nice response curves of the kit set,
and it was not good enough to give selectivity between locals
here where I wanted to hear a 300 watt station which was only
45 kHz away from a 5,000 watt station.
But otherwise, the TRF was a fine performer.



So what it boils down to is that you haven't tried a wideband 455 kHz
filter while I have, and I haven't tried a 2.0 MHz IF filter, which you
may or may not have done.

I have tried getting 455 kHz IFTs to go wider, but I was dissapointed with
overall results, because I'd have needed 3 IFTs, and lots of damping.

I got 10 kHz of audio BW at low thd using simple methods of damping, sliding IFT1
coils closer,
and some RC boosting of audio HF.
I thus achieved the use of tubes, good AF BW, and excellent local station
selectivity, which allowed me to hear my wanted 300 watt station without
the 5,000 watt station able to be heard even though it is only 45 kHz away.

I at least have cited some concrete facts about
IF filters, while you have only muttered about Q, without indicating how
it actually relates to the problem. I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

I leave you to wonder the full content of my mutterings,
and I do hope you spend some time soon in your shack with a soldering iron
and response meter.

The other advantage of a 2 MHz IF is that the filtering of RF from the recovered
audio is easier, because the C value is less, and the filter used has less effect
on recovered audio at 10 kHz, and at high amplitudes.

But don't let me mention it, I know you'd be aware of it already.

Patrick Turner.



Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/